Voters in Serbia are casting ballots in a presidential election seen as a test of public support for populist Prime Minister Aleksandar Vucic and his autocratic rule.

Mr Vucic, an ultranationalist turned pro-European Union politician, is tipped to win by a high margin against 10 opposition candidates.

His political clout could face a blow, however, if he does not see off his opponents in the first round of voting on Sunday.

Mr Vucic needs to win by more than 50% of the vote to avoid a run-off election on April 16 that would put him in a much trickier position against a single opposition candidate.

Mr Vucic - who has been prime minister since 2014 - is expected to use his predicted win to appoint a figurehead successor and transform the presidency from a ceremonial office into a more muscular role.

AP

Contrary to his claims that he wants to lead Serbia into the EU, Mr Vucic has been pushing for deeper ties with long-time ally Russia.

Right before the vote, Mr Vucic even visited Russian president Vladimir Putin, who reportedly promised his signature on the delivery of fighter planes, battle tanks and armoured vehicles to Serbia.

The move triggered fears of an arms race in the western Balkans, which Russia considers its sphere of influence.

Mr Vucic's main challengers in the vote include human-rights lawyer and former Ombudsman Sasa Jankovic, former foreign minister Vuk Jeremic and firebrand nationalist Vojislav Seselj, who has been tried for war crimes.

One of the biggest surprises of the election campaign has been Luka Maksimovic, a media student who is running as a grotesque parody politician, decked out in a white suit, oversized jewellery and a man-bun.

Mr Maksimovic's parody character mocks corrupt Serbian politicians by promising to steal if he is elected.

His widely-viewed videos on social media networks portray him doing push-ups, sucking a raw egg or riding a white horse surrounded by mock bodyguards.

His supporters are mostly young voters alienated by Serbia's decades-long crisis and economic decline.

The opposition has accused Mr Vucic of muzzling the media and intimidating voters ahead of the election.

Mr Vucic denies such accusations, saying only he can bring stability to a region scarred by the wars of the 1990s.